Pancreatic cancer (PC) is the 3rd most common cause of cancer deaths in the United States with a dismal 5- year overall survival of 8%. There are limited treatment options for individuals diagnosed with PC even if detected early. As a result, there is a vital need to improve existing therapies or develop new strategies to increase the overall survival of PC patients. An emerging strategy called stereotactic body radiotherapy (SBRT), where higher doses of radiation are delivered over a short period of time, has demonstrated superior tumor control when compared to conventional radiotherapy. This application will expand on these findings and incorporate a new paradigm shift in SBRT that overwhelmingly demonstrates a crucial role of the immune system in mediating the anti-tumor effects of this modality. Therefore, our overarching hypothesis is that SBRT efficacy can be enhanced by modulating the immune response in PC. To test this, we developed two innovative approaches: First, we activated antitumor immune cells by delivering the immunostimulatory cytokine interleukin-12 (IL-12) using a cutting-edge technology called microspheres (MS). These polymers encapsulate IL-12, and when injected intratumorally, provide a slow, continuous release of cytokine for 10-14 days. Second, a preclinical orthotopic model of PC was established where pancreatic tumors were treated with a clinically relevant schedule of SBRT using a recently acquired Small Animal Radiation Research Platform (SARRP); an instrument that closely resembles those used in clinical radiation oncology. When SBRT was combined with IL-12MS, an unprecedented decrease of tumor burden, and even cure, was observed in 2 different orthotopic PC models. This proposal will build on these encouraging studies and address mechanisms of action both at the primary tumor (Aim 1) and at the predominate site of PC metastases, the liver (Aim 2). In Aim 1, we will determine if SBRT + IL-12MS can convert a typically immunosuppressive tumor microenvironment (TME) into one that is immunostimulatory using 2 separate orthotopic and one spontaneous PC model. We predict that SBRT + IL-12MS augments the antitumor capacity of CD8+ T cells along with repolarizing suppressive tumor- associated macrophages (TAMs) into cells that either directly or indirectly bolster tumor destruction. In Aim 2, we will explore whether localized SBRT + IL-12MS therapy to the pancreas modulates the hepatic microenvironment to destroy metastases. We predict that localized SBRT + IL-12MS potentiates systemic antitumor immunity, which in turn conditions the hepatic microenvironment by upregulating the cytokine IFN? and protects against the establishment of metastases. This would have important clinical significance as many PC patients succumb to metastatic disease. In summary, the grant proposed here will obtain the necessary pre- clinical data that will ultimately inform the use of this exciting new PC therapy, SBRT + IL-12MS, in an investigator led clinical trial.